Added Stock query API.

Warning : at this point, only getInventory is fully working. Other methods like
getAvailableInventory, getInventoryList ... will be rewritten soon.


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@2740 20353a03-c40f-0410-a6d1-a30d3c3de9de